A recently developed type of secondary or rechargeable electrical conversion device comprises: (1) an anodic reaction zone containing a molten alkali metal anode-reactant, e.g., sodium, in electrical contact with an external circuit; (2) a cathodic reaction zone containing (a) a cathodic reactant comprising a liquid electrolyte, e.g., sulfur or a mixture of sulfur and molten polysulfide, which is electrochemically reversibly reactive with said anodic reactant, and (b) a conductive electrode which is at least partially immersed in said cathodic reactant; and (3) a solid electrolyte comprising a cation-permeable barrier to mass liquid transfer interposed between and in contact with said anodic and cathodic reaction zones. As used herein the term "reactant" is intended to means both reactants and reaction products.
During the discharge cycle of such a device, molten alkali metal atoms such as sodium surrender an electron to an external circuit and the resulting cation passes through the solid electrolyte barrier and into the liquid electrolyte to unite with polysulfide ions. The polysulfide ions are formed by charge transfer on the surface of the porous electrode by reaction of the cathodic reactant with electrons conducted through the porous electrode from the external circuit. Because the ionic conductivity of the liquid electrolyte is less than the electronic conductivity of the porous electrode material, it is desirable during discharge that both electrons and sulfur be applied to and distributed along the surface of the porous conductive material in the vicinity of the cation-permeable solid electrolyte. When the sulfur and electrons are so supplied, polysulfide ions can be formed near the solid electrolyte and the alkali metal cations can pass out of the solid electrolyte into the liquid electrolyte and combine to form alkali metal polysulfide near the solid electrolyte.
During the charge cycle of such a device when a negative potential larger than the open circuit cell voltage is applied to the anode the opposite process occurs. Thus electrons are removed from the alkali metal polysulfide by charge transfer at the surface of the porous electrode and are conducted through the electrode material to the external circuit, and the alkali metal cation is conducted through the liquid electrolyte and solid electrolyte to the anode where it accepts an electron from the external circuit. Because of the aforementioned relative conductivities of the ionic and electronic phases, this charging process occurs preferentially in the vicinity of the solid electrolyte and leaves behind molten elemental sulfur.
It has been customary to prepare cells or batteries of this type without regard to the presence of corrodable materials or other impurities, particularly in the presence of the cathodic reactants. Thus, the cell or battery container may have been made of metal, e.g. stainless steel, and the porous electrode may have been formed of metal, e.g., a stainless steel felt. Although such cells demonstrate excellent rechargeability characteristics initially, they tend to show decreased capacity with each successive cycle. An examination of such prior art cells that have deteriorated in their charge/discharge capacity to the point of failure has shown that the shortened cycle life and deterioration of charge/discharge capacity of these cells might be attributed, at least in part, to the corrosion of the metal container or electrode, the accumulation of metal corrosion products on the solid electrolyte surface, the accumulation of corrosion products within the porous electrode and decreased mobility of sodium polysulfide within the porous electrode as a result of such corrosion product accumulation.
In order to solve the problems of decreased cycle life and deteriorating charge/discharge capacity it has been proposed to prepare the cell or battery so as to exclude metal or metal compound impurities, particularly corrosion products resulting from polysulfide attack on metal cell parts. The absence of such impurities can be reasonably assured by employing both an electrode and a container of such a nature that it will not contaminate the cathodic reactant during operation of the cell or battery. The exclusion of corrodable surfaces from the cell or battery, particularly from the cathodic reaction zone may be accomplished in numerous ways. Thus, the container and electrode may be formed from inherently noncorroding, conductive materials, or they may be treated in some way so as to render them noncorrodable, but still conducting. Also, the container could be formed from noncorroding, nonconductive materials such as glass or ceramic and electrical contact between the electrode and the external circuit made by current lead through rather than through the container body. Another possibility would be to provide the container with a noncorroding, protective liner which may or may not be conductive. Still further ways of excluding corrosion products from the cell may be apparent to those skilled in the art. Those mentioned above are merely exemplary of precautions which may be taken and do not constitute a part of the invention described in this application.
Preparing cells or batteries free of metals or metal compound impurities does result in a significant increase of charge/discharge cycle life and a stabilization of charge/discharge capacity. However, such cells, after initial discharge, do not recharge to the extent that those cells containing impurities do. Thus, these cells exhibit reduced charge/discharge capacity.
The improved cell or battery of this invention overcomes the problems of lack of capacity found in the metal-free or noncorroding cells and yet retains many of the benefits of such cells such as long cycle life and stabilized capacity.